It is known to use a femtosecond laser in a method in order to activate, using the optical pulses thereof, a photoconductive dipole antenna. In the dipole antenna the laser pulses generate free charge carriers, which are accelerated by an external electrical field. The accelerated charge carriers, in the form of a short current pulse, form the source for an electromagnetic pulse radiated into the space. The electromagnetic pulses generated in this manner may, for example, be used for investigating material, these being used in turn, for example, for monitoring during the production processes of plastics material products or to analyze material. The generation of two time delayed pulses is also the basis for further fields of application such as pump-probe methods, optical tomography, interferometric measurements etc.
These methods are most widespread in the form where a pulsed beam is generated using a beam source, in particular a laser. This beam is divided into a first partial beam with a first pulse and a second partial beam with a second pulse and the pulses are then directed into a target. In this case, one of the two pulses covers a path which is different from the second pulse. This delay path can be realized in the most varied ways, such as, for example, by a mirror, which opposes a second mirror, the spacing of the two mirrors being controllable by means of a precise mechanism or an electronic system. An injected, pulsed beam, in a structure of this type, can cover an adjustable path, which is called a delay path. This path is used to adjust the time delay from a pulse decoupled before this delay path. In this case, a main problem consists in permanently adjusting the mechanical arrangement of the delay path and it is known from constructional systems in this regard that they frequently have to be readjusted.
It is furthermore known from the prior art to configure a delay path for optical signals with a decoupling mirror, which has a spirally curved mirror face. DE 10 2005 011 045 A1 in this regard shows a mirror face with a spiral curvature, the radius changing with the change in the angle of rotation. If a mirror of this type is installed in a corresponding arrangement, the delay path can be changed by rotating the mirror. The mirror body, for this purpose, has two mirror faces, which are arranged symmetrically with respect to a center point, each partial face, starting with an increasing radius, running from a minimum radius through to a maximum radius. Similar problems are produced here as in the previously mentioned prior art in that a mechanism, in this case a rotary mechanism, is required to adjust the delay path, which has to be very precise, and in that the delay path is limited with regard to its variability depending on the radius.
In the conventional method, in particular in the method described above to generate time delayed pulses, the time delay is coupled to the length of the delay path. A limitation of the time delay is thereby produced in that the length of the delay path is limited by the size of the structure. Because of the mechanisms used and the component geometries used, the variability of the delay paths in constructional systems of this type is also very limited. With regard to the delay paths used, problems are also produced with regard to the readjustment and imprecisions due to vibrations at the structure, as with longer optical path lengths covered, the lateral deviations in the sense of the beam laws increase proportionally thereto.